Highly precise alignment is required in the photolithographic processes utilized in semiconductor device manufacturing. For example, fabricating semiconductor devices typically requires a series of photolithographic masks used in sequence to create patterns on a semiconductor substrate. Each mask must be positioned precisely with respect to the semiconductor substrate and with respect to the location of the previously used masks.
A variety of methods have been employed in the past in order to precisely locate photolithographic masks and semiconductor substrates. For example, diffraction gratings have been formed in the substrate. The grating is scanned by a light beam and the position of the substrate is detected based on the diffraction of the light. Alternatively, an optically detectable "target" is patterned on the substrate or mask, the target's position being precisely known with respect to the rest of the substrate or mask. An image of the target is captured and the center or edges of the image is located, thereby revealing the position of the substrate or mask. However, such conventional methods are extremely sensitive to anomalies on the surface of the object being positioned. For example, asymmetric photoresist coatings or grains of processing coatings such as metal grains will negatively impact both the diffraction grating alignment method and the conventional optical target location method. For example, with respect to diffraction grating methods, such anomalies will scatter light in an unpredictable way. With respect to the conventional optical target imaging methods, such anomalies will form part of the image, making the image asymmetric. Consequently, the "center" of the image will be offset in an unpredictable way.
What is needed is a method for alignment in photolithographic processing which is robust and insensitive to anomalies on the surface of the object to be positioned.